By the time of the European Conquest, squash of up to 200 lbs (90 kg) were common in South America. It may sound like a long way from 200 lbs to the 1,810.5 lbs (821.23 kg) of today’s World Record-holder, but it has taken a remarkably short amount of time. Many factors had to come together.

First, the plants had to be moved out of the tropics. Although the growing season is much longer in the tropics, day length is short and the large fruits are subjected to many pests and diseases over an extended growing period. At our latitude the fruits can grow quickly during the long summer days. This first step occurred in the 1700s when C. maxima fruits from South America reached New England and Europe.

Next, selective breeding specifically aimed at growing larger fruit took place. This began in earnest a little over a hundred years ago when more diversity became available and different cultivars could be crossed. These large fruits became curiosities, most notably in 1893 where a 365-pounder from Ontario, grown by William Warnock was exhibited at the Chicago World’s Fair. In 1900, Warnock took the prize at the World’s Fair in Paris with the first 400 lb pumpkin.

For the next 70 years, little changed in this maximum weight. Finally, in the 1970s and 80s, records started being smashed … I mean squashed, when Howard Dill of Nova Scotia bred the ‘Atlantic Giant’ pumpkin, which is the basis for all giant pumpkins today.

The third critical factor in achieving larger pumpkins was an improvement in cultural practices. It has been known since at least William Warnock’s time that to achieve maximum size, only one fruit per vine should be allowed to ripen, but many other improvements are still being discovered and tweaked concerning how to treat the soil, leaves, and fruit throughout the growing season. Best practices in cultivation are a constant source of discussion and debate among passionate giant pumpkin growers.
-https://www.nybg.org/blogs/plant-talk/2010/10/garden-programming/origin-of-the-giant-pumpkin/

History:
Though large, Thoreau’s pumpkin hardly came close to the world record for 1857. That distinction went to a grower in southwest England whose fruit weighed in at 245 pounds. Other records followed over the years, but the watershed moment came from William Warnock, a machinist and farmer from Goderich, Ontario. In 1893, he produced a 365-pounder for the Chicago World’s Fair; seven years later, in Paris, his entry weighed 400 pounds. His next world record—403 pounds at the 1904 St. Louis World’s Fair—would hold for more than 70 years. “For exhibition purposes, it stands without a rival,” the 1924 Rennie Seed Company catalog noted of the lineage: “Skin dark green, flesh golden yellow.”

Warnock’s record was finally shattered in 1976 by a Pennsylvania grower, but it was a Canadian named Howard Dill who ushered in modern competitive gardening. Dill spent 30 years crossing Mammoth pumpkin varieties with one another, trying to isolate the best characteristics, such as a rich orange color. Beginning in 1979 Dill grew the world’s biggest pumpkin four years in a row, and he landed in the Guinness Book of World Records in 1981 for a 493.5-pounder. Today’s growers still use seeds descended from “Dill’s Atlantic Giant,” a variety he registered with the U.S. Department of Agriculture’s plant variety protection office in 1986. While other fruits, including the field pumpkin, long gourd and watermelon, have put on some serious pounds in recent years, none has matched the Atlantic Giant, which sets a new record nearly every year.
Read more: https://www.smithsonianmag.com/science-nature/the-great-pumpkin-74423844/#xivGmbKFpIkYROYQ.99
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Dill's Seed History:
Atlantic Giant Pumpkins were developed by Howard Dill Enterprises in Windsor, Nova Scotia, Canada. They were developed specially for pumpkin size competitions, from a cross between Goderich Giant, Genuine Mammoth Pumpkins and Mammoth Tours. Development took place from 1973 to 1979.
-cooks info

Edible:
The pumpkin has thick flesh walls.

Some say Atlantic Giant are also good for pies. Others say that if by “good” people mean “edible”, it is, but that it doesn’t have much flavour at all.
“By God, if we can get a pumpkin up to a ton, imagine what we can do to somebody’s vegetable crop,” says Stelts, president of the Great Pumpkin Commonwealth, which oversees official weigh-offs. “What we are doing will be reflected on the dinner table of America.”

Contests:
In 2000, instead of driving his pumpkin to a weigh-off in New York State and netting a $10,000 bonus, he decided to stay in Ohio, where the prize money was only $1,500. “Not to be able to share it with all my friends would have been a crying shame,” he says.
The Ohio Valley contest, Werner’s local weigh-off, is one of more than 80 competitions in the “Great Pumpkin Belt,” which stretches across North America from Washington State to Nova Scotia. This is prime pumpkin territory—offering 90 to 120 frost-free summer days, but cold enough in winter to keep plant diseases and pests in check. The weigh-offs are friendly competitions, but they’re also a form of citizen science, with growers meticulously graphing their pumpkins’ growth curves and sharing success and failure with their peers.
The path to prizewinning pumpkins can be traced, improbably, to Henry David Thoreau. In the spring of 1857, while living in Concord, Massachusetts, Thoreau planted six seeds from a French variety called Potiron Jaune Gros de Paris (fat yellow Paris pumpkin). He was astonished that fall when one fruit reached 123.5 pounds. “Who would have believed that there were 310 pounds of Potiron Jaune Grosse in that corner of my garden!” he wrote in Wild Fruits.
There have been contests since at least 1857, where a grower from England won with a 245 lb pumpkin.

Growing:
Werner follows Warnock’s chicken-manure prescription, hauling out about 1,000 pounds every spring, but he’s more scientific. He rotates his pumpkin patch, growing sorghum in the summer in a patch he’s preparing for the next year. He plows under a winter crop of rye before planting his pumpkins. Both grasses have bacteria that pull nitrogen from the air and convert it to ammonia, enriching the soil. And as the vines creep along the bare ground in early summer, he scoops up a sandwich-bagful of dirt, plucks a few leaves and FedExes the material to John Taberna at Western Laboratories in Parma, Idaho. After Taberna told Werner that his pumpkins lacked magnesium and manganese, Werner began spraying them with a chelated fertilizer. Werner also adds his own microorganisms to the soil.
Scientists have long recognized the degree to which plants depend on microbes to obtain nutrients, but that knowledge has been applied only in limited ways in agriculture. In areas that have been devastated by wildfires or strip mining, some government agencies spray mycorrhizal fungi on seedlings or blend it into the soil to improve tree survival and growth. The practice broke into competitive pumpkin growing in 2005, when a Rhode Islander named Ron Wallace phoned Reforestation Technologies International, a Salinas, California, plant nutrient company, and asked to test its commercial mycorrhizal product. “I’ll give you 20 pounds, but if you win any prizes, I want bragging rights,” said company president Neil Anderson. Sure enough, Wallace went on to break the pumpkin world record in 2006, and Anderson began marketing Xtreme Gardening products a few years later, to which he recently added the nitrogen-fixing bacteria Azospirillum. “Bacteria are miniature fertilizer factories,” he says.
plants have two types of tissue that work to get food and water flowing through them: xylem and phloem. The xylem transports water into the plants, and the phloem is responsible for sugar movement. While all pumpkins easily move large amounts of water, Savage found that giant pumpkins have supersized phloem.
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Size
All of which raises the question of just how much larger they can get. “Nobody knows what the limit is going to be,” says Andres, of the New York Botanical Garden. In fact, mechanical engineer David Hu and colleagues at the Georgia Institute of Technology have been investigating pumpkin growth. A world-record strawberry or tomato weighs about ten times the average, they found. By contrast, giant pumpkins weigh 100 times the average. And Hu thinks they can get even bigger. To figure out how much bigger, he and his colleagues placed pumpkins of various sizes in a vise-like instrument and subjected the fruits to pressure until they cracked. These force measurements led them to estimate just how big a pumpkin might get in a perfect world. The answer: 20,000 pounds. Of course, real pumpkins with their warts, scars and dimples are unlikely to ever come close to geometrical perfection. A 1,000-pound pumpkin may have a wall that’s 16 inches thick on one side and one inch on the other, a recipe for disaster, or at least a very large pumpkin pie.